Chair base



FIG. 1 is a front perspective view of a chair base showing my new design;

FIG. 2 is a front elevational view thereof, the rear, right and left elevational views being the same;

FIG. 3 is a front-right elevational view thereof;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof.

The elements illustrated in long and short broken lines shown in the drawings illustrate portions of the chair base and form no part of the claimed design. 

CLAIM The ornamental design for a chair base, as shown and described. 